In transmission systems for heavy-duty vehicles, for example trucks and buses, it is known to connect an extra gearbox to the basic gearbox of the vehicle for the purpose of doubling the number of gearing possibilities (see, for example, U.S. Pat. No. 4,821,591 or WO 9620359). Such a gearbox is usually referred to as a range gearbox or an auxiliary gearbox. The range gearbox usually comprises (includes, but is not necessarily limited to) a planetary gear, by means of which the gearing can be changed between a high-range mode and a low-range mode. In the low-range mode, use is made of the gearing in the planetary gear, whereas, in the high-range mode, no transmission takes place through the planetary gear. In order to facilitate shifting between high-range mode and low-range mode, it is known to design such planetary gears with synchronizing means, usually comprising synchronizing rings, spring means for pre-synchronizing and blocking means in order to prevent engagement before synchronous rotational speed has been achieved.
In order to reduce the number of component parts and also from the point of view of power distribution, it has been found to be advantageous to use the ring gear of the planetary gear as a coupling sleeve. The ring gear can, by axial displacement and after synchronization of the rotational speed difference between the ring gear and coupling rings, be brought into engagement with the coupling ring concerned on either side of the planetary gear, and, in this way, different gearings are obtained. Examples of known designs having an axially displaceable ring gear are shown in W00155620. In this case, the ring gear is provided at each axial end with synchronizing means, and the internal teeth of the ring gear are extended so as to be capable of being coupled together with a corresponding coupling ring with coupling teeth arranged on each side of the ring gear. The ring gear and the synchronizing means on each side thus occupy more space in the axial direction.
When the ring gear in W00155620 is coupled together with the gearbox casing, low-range mode is obtained and the gearing in the planetary gear is used.
In order to obtain a shorter shifting time when synchronizing the rotational speed for the low-range mode, a greater synchronizing torque is required compared with synchronization for the high-range mode. A large low-range synchronization diameter is thus desirable in order to obtain a greater synchronizing torque. The diameter refers to the position of the synchronizing friction surfaces. In the range gearbox according to W00155620, the inside diameter of the ring gear limits the possibility of a large synchronizing diameter and thus a great synchronizing torque. Another disadvantage of known art is that the overall length is in many cases too great.